Theoretical Study of the Effect of Lewis Acids on Dihydrogen Elimination from Niobocene Trihydrides

Abstract

Dihydrogen elimination from polyhydride metallocene complexes is usually a process of high-energy cost. In this paper we study, from a theoretical point of view, the effect in the loss of dihydrogen from the niobocene [Cp 2 NbH 3 ] complex upon addition of three different Lewis acids (in increasing order of acidity: HBO 2 C 2 H 2 as a model of catecholborane, BF 3 , and BH 3 ). Our DFT calculations show that the Lewis acid can interact either with the central or the lateral hydride leading to two different minimum-energy adducts. The lateral adduct is still a dihydride complex in the HBO 2 C 2 H 2 case, whereas it shows a clear dihydrogen structure in the other two cases. This adduct is the one that leads to dihydrogen elimination. The transition states for this process show that the stronger the Lewis acid, the lower the energy barrier. In all the cases the Lewis acid favors the dihydrogen elimination process as compared with the noncatalysed H 2 elimination from the niobocene trihydride. The products are also greatly stabilized as the presence of a HBR 2 Lewis acids allow the final complex to remain coordinatively saturated upon formation of an h 2 -BH 2 R 2 complex. In BF 3 this complex cannot strictly be formed, but here the fluorine plays the role of the missing hydrogen. Finally, the implications of the different energy profiles in the kinetics of the whole process are discussed.

Quantum Mechanical Exchange Coupling in Polyhydride and Dihydrogen Complexes

“…22 A theoretical study was performed on these d 0 complexes and led to the same conclusions as the ones obtained for the iridium d 4 compound (see section III.A.3). 25 This system reflects the general trends found throughout the complexes displaying exchange couplings: cationic systems display higher couplings than analogous neutral ones and second-row transition metals higher than third-row transition metals. In this respect, it was interesting to look at cationic third-row transition metals.…”

“…More recent calculations using the DFT method reproduced satisfactorily the trends of exchange couplings in a series of metallocene trihydrides including Cp 2 NbH 3 and Cp 2 TaH 3 . 25 The authors propose the involvement of a dihydrogen-like transition state and that the major factor governing the magnitude of exchange couplings is the stability of the dihydrogen configuration relative to the minimum energy trihydride.…”

Density Functional Study of a d2-C5H5Nb(butadiene)R+ Ethene Polymerization Catalyst

“…This scheme has been previously utilized in similar calculations. 9,11, [29][30][31] Basis sets of this and lower level (on the active part) have been successfully applied to study analogous catalytic processes on Zr-systems. 5,9,30 The third basis (B3) is the same as B2 augmented with a set of f-type Gaussians with an exponent R ) 0.952 32 on niobium.…”

11 Vanadium, niobium and tantalum

“…82 A theoretical study of the effect of Lewis acids on dihydrogen elimination from [NbH 3 Cp 2 ] has been carried out. 83 Syntheses and characterisation, including crystal structures, have been reported for [M(4-supine-p-C 6 H 4 OMe-dad)(Z 5 -C 5 R 5 )] (M Nb, Ta; R H, Me) and related compounds. 84 The crystal structure has also been reported for [Ta(s-trans-Z 4 -butadiene)Cp*Cp] .…”